Imaging apparatus that utilize relatively high energy radiation such as x-ray and gamma rays are widely used to obtain images of subject matter more or less opaque to electromagnetic energy in the visual spectrum. For example, x-ray imaging technology has been employed in a wide range of applications from medical imaging to detection of unauthorized objects or materials in baggage, cargo or other containers. X-ray imaging typically includes passing high energy radiation (i.e., x-rays) through an object to be imaged. X-rays from a source passing through the object interact with the internal structures of the object and are altered according to various characteristics of the material (e.g., transmission, scattering and diffraction characteristics, etc.) which the x-rays encounter. By measuring changes in the x-ray radiation (e.g., attenuation, modifications to the energy spectrum, scatter angle, etc.) that exits the object, information related to characteristics of the material, such as the density distribution, may be obtained.
Computer tomography (CT) techniques involve capturing transmitted x-ray information from numerous angles about an object being imaged to reconstruct a three-dimensional (3D) volume image of the object. The data obtained from each view angle is referred to as projection data or view data and is indicative of the absorption characteristics of the object in directions related to the respective view angle. CT imaging often involves obtaining hundreds or thousands of projections to form a 3D reconstruction of the projection data, thus requiring the object to be exposed to relatively large doses of x-ray radiation and/or to (large or small) doses of radiation over relatively long exposure times. Such large doses and/or imaging times may not be suitable for certain imaging applications having particular safety and/or time constraints. For example, when imaging human tissue, and/or when the imaging procedure is performed on a routine or frequent basis (such as is often the case in mammography), dose levels and/or exposure times used in conventional CT imaging may exceed that which is more desirable.
To reduce a patient's exposure during breast imaging procedures (e.g., imaging of the human female breast), conventional mammography is often performed by obtaining only a pair of two-dimensional (2D) radiographic images of the breast (i.e., each image is reconstructed from a single projection of the breast), typically acquired at approximately complementary angles to one another. However, the superposition of structure within the breast that occurs when 3D structure is projected onto two dimensions often obscures the true nature of the structure. This superposition of structure may make it difficult to identify or detect tissue anomalies. For example, distinct structure in 3D that overlaps in 2D may make it difficult to distinguish cancerous subject matter from benign subject matter within the breast.
Thus, conventional approaches to providing generally low-dose radiation imaging suffer from images that provide confusing representations of internal structures of an object due, at least in part, to the projection of three-dimensional structure onto one or more two-dimensional images. The resulting superposition of distinct structure located at different levels in 3D makes discerning the actual structure in a 2D representation difficult, rendering conventional imaging procedures vulnerable to errors in diagnosis. In mammography, the inability to ascertain the true nature of breast structure may result in both significant false negative and false positive rates, leading to potential missed early stage cancers in the case of the former, or unnecessary trauma to the patient and/or unnecessary hospital visits, surgical procedures, etc., in the case of the latter.
To address such problems, it has been proposed to use a selected number of projections obtained from a plurality of view angles to reconstruct a 3D image, while still respecting a relatively low dose budget (e.g., dose budgets suitable for mammography or other tissue exposures that are generally dose limited due to safety concerns). U.S. Pat. No. 6,744,848 (hereinafter the '848 patent), entitled “METHOD AND SYSTEM FOR LOW-DOSE THREE-DIMENSIONAL IMAGING OF A SCENE,” describes various methods and apparatus for obtaining 3D images in a relatively low dose environment. In addition, U.S. Pat. No. 5,872,828 (hereinafter the '828 patent), entitled “TOMOSYNTHESIS SYSTEM FOR BREAST IMAGING,” describes various methods of reconstructing projection data from a generally limited number of view angles to form a 3D image. Both the '848 and '828 patents are herein incorporated by reference in their entireties.